The semiconductor integrated circuit (IC) industry has experienced rapid growth. Technological advances in IC materials and design have produced generations of ICs. Each generation has smaller and more complex circuits than the previous generation.
Size reduction of the metal-oxide-semiconductor field-effect transistor (MOSFET) has enabled the continued improvement in speed performance, density, and cost per unit function of integrated circuits over the past few decades. As the MOSFET scaling down process continues, the area of the gate dielectric layer of the MOSFET is scaled down as well. The relationship between the gate capacitance (C), the dielectric constant (∈, which is also called the permittivity), the area (A) and the physical thickness (T) of the gate dielectric layer are shown in Formula (1):
                    C        =                                            ɛ              ×              A                        T                    .                                    Formula        ⁢                                  ⁢                  (          1          )                    
As shown in Formula (1), the scaling down of the area (A) of the gate dielectric layer reduces the gate capacitance (C). Since the performance (i.e. drain saturation currents) of the MOSFET is proportional to the gate capacitance (C), the performance is reduced as the gate capacitance (C) is reduced.
The gate capacitance and the performance may be maintained by the scaling down of the thickness of the gate dielectric layers. However, the scaling down of the gate dielectric thickness results in a large gate leakage current.